It's an 800MHz FSB Pentium Xeon model, so it is an EM64T 64-bit capable processor. RHEL 5 32-bit or 64-bit will install fine on it, regardless of whether it is a SCSI or SATA model.

The standard questions apply, though. This is an old server model. Do you intend to use RAID? Also, why are you specifically looking at the 5.5 release of RHEL? Version 5.8 is available as of this date.